APware

Overview

The performance
seen by individual clients on a wireless local area network (WLAN) is
heavily influenced by the manner in which wireless channel capacity is
allocated. The popular MAC protocol DCF (Distributed Coordination
Function) used in 802.11 networks provides equal long-term transmission
opportunities to competing nodes when all nodes experience similar
channel conditions. When similar-sized packets are also used, DCF leads
to equal achieved throughputs ( throughput-based fairness) among
contending nodes.

Because of varying indoor channel conditions, the 802.11 standard
supports multiple data transmission rates to exploit the trade-off
between data rate and bit error rate. This leads to considerable rate
diversity, particularly when the network is congested. Under such
conditions, throughput-based fairness can lead to drastically reduced
aggregate throughput.

We argue the advantages of time-based fairness, in which each competing
node receives an equal share of the wireless channel occupancy time. We
demonstrate that this notion of fairness can lead to significant
improvements in aggregate performance while still guaranteeing that no
node receives worse channel access than it would in a single-rate
WLAN. We also describe our algorithm, TBR (Time-based Regulator), which
runs on the AP and works with any MAC protocol to provide time-based
fairness by regulating packets. Through experiments, we show that our
practical and backward compatible implementation of TBR in conjunction
with an existing implementation of DCF achieves time-based fairness.

Furthermore, in a series of experiments, we demonstrate that the DCF
encourages non-cooperative nodes to use globally inefficient transmission strategies that lead to degraded
aggregate throughputs. This is because in non-cooperative environments
such as public hot-spots, individual nodes attempt to maximize their
achieved throughput by adjusting the data rate or frame size used,
irrespective of the impact of this on overall system performance.

We show that by establishing independence between the
allocation of the shared channel time and the strategies used by
individual nodes, an improved MAC protocol can lead rational but
non-cooperative nodes to make choices that increase aggregate
throughputs by as much as 30% under some conditions.

Current Status

We have implemented TBR using Linux PCs equipped with standard PRISM II-based
802.11b wireless interfaces. The system allows long-term capacity
allocation of competing nodes according to a desired allocation
vector. Our experimental results agree with our analytical results that
time-based fairness can significantly improve the performance of
multi-rate WLANs.